This invention relates to a radiation curable primer coating composition suitable for use in forming a polyolefin coating on a steel substrate.
Polyolefin coated steel pipes have been widely used as pipelines for conveying petroleum or natural gas. Since most of these pipelines are subjected to severe environmental conditions, high levels of adhesive and anticorrosive characteristics are required for the polyolefin coating layer.
Polyolefin resins are nonpolar and have crystalline nature and thus exhibit excellent chemical stability and excellent mechanical properties. However, their adhesion strength to a steel substrate are generally poor when polyolefin coatings are applied directly. It is for this reason that an adhesive layer consisting of a modified polyolefin such as ethylene-acrylic acid copolymers or ethylene-maleic anhydride copolymers is interposed between the polyolefin protective layer and the steel substrate. This modified polyolefin layer exhibits an excellent adhesion strength to the substrate but this adhesion strength tends to decrease the coated products are exposed to severe corrosive environment. In order to avoid this usually epoxy based primers are applied on the substrate surface. Thus the entire operation for producing polyolefin coated steel pipes include the steps of polishing the substrate surface to remove oxide films using such techniques as sand blasting or shot blasting, subjecting the substrate to a surface treatment such as chromate treatment if necessary, applying the epoxy primer, and curing the primer by heating to a temperature from 130.degree. C. to 200 .degree. C. using an induction heater or hot air oven. After the applied primer coating has been fully cured so that the coating film is not easily scratched or peeled off when contacted by conveying rolls, the pipes are passed to next conveying rolls for extrusion coating the interposing adhesive layer and the polyolefin outer layer successively.
Epoxy primer compositions currently used for this purpose require heating to a temperature above 130.degree. C. for about 3 to 4 minutes. This limits the productively of polyolefin coated steel pipes when the coating is carried out while continuously conveying pipes axially. In order to convey substrate pipes smoothly, the distance between adjacent conveying rolls should be less than one half of the pipe length. Assuming that the distance between adjacent conveying rolls is 3 m and a curing time of 3 minutes is required for curing the epoxy primer coating, the conveying speed is limited to less than 1 m/minute.
In addition to low productively, a substantial amount of heat energy is required for heating steel pipes.
It is therefore a principal object of the present invention to provide a primer coating composition free from these disadvantages for use in improving the adhesion strength of polyolefin layers to a steel substrate.
It is another object of the present invention to provide a primer coating composition of the above-mentioned type which cures within a very short length of time by irradiating with actinic radiation.
Other objects and advantages of the present invention will become apparent as the description proceeds.